Question 1: What are the OSPF Loop Prevention Mechanisms?
In single area, the routers have the Link State Database, having the same LSDB helps the routers to build a loop-free topology.
Now in a multiarea topology, the ABRs are responsible for advertising an intra area route into another area as an inter-area route, and because the area 0 is the backbone that connects all other areas, therefore the developers of OSPF designed the ABR as the responsible of avoiding the routing loop between areas, the rule states that when an ABR learns an LSA Type-3 from another ABR through a non-backbone area, it does not trust this Type-3, in other words it ignores it for SPF calculation.
Now what about the NSSA area, of course there is a routing loop prevention mechanism, this mechanism is based on the P-bit and it is used when two NSSA ABRs are advertising a default Type-7 using the area x default-information originate command, the P-bit in the default Type-7 LSA is cleared so that another NSSA ABR will not install a default route in its routing table, this is what we call the P-bit loop prevention mechanism.
RFC Loop mechanism, another way to ensure that the routing loop is avoided is to be sure you are using the same RFC either 2328 or 1583 on all routers in a multiarea design, otherwise a loop can occur between an internal router and an ABR because the rule of path selection differs between RFC 2328 and RFC 1583. RFC 2328: an intra-area route through a non-backbone is always preferred than an intra-area route via backbone or any inter-area route. And the rule of RFC2583: the path selection is based on the cost when comparing two intra-area routes regardless the area from they are learned.
The last mechanism is when you have two regular ASBRs connected to the same area and advertising a default route using the default-information originate command, when an ASBR receives a Type-5 LSA for a default route and it already originated a default Type-5 using the same command, it ignores this default route, this is what i call ASBR default route loop prevention mechanism.
Question 2: Why area 0 is required?
Per RFC 3509 two non-backbones area cannot communicate and to ensure a loop free topology in a multi area design, especially with redundant paths, we need a central point and a reference to calculate the best path without a risk of routing loop, here comes the ABR which a special router that interconnects a non-backbone area with a backbone area, all inter-area traffic must go through the backbone area.
OSPF is a link-state routing protocol only within an area (intra-area); but almost a distance-vector routing protocol between areas (inter-area).
One of the advantages of link state protocols is that the link state database provides a “view” of the entire network but only within the area. Within the same area every OSPF router floods information about itself, its links, and its neighbors to every other router. From this flooded information each router builds an identical link state database. Each router then independently runs a shortest-path-first calculation on its database and calculates the best path to each destination.
When an OSPF domain grows large, the flooding and the resulting size of the link state database becomes a scaling problem. The problem is remedied by breaking the routing domain into areas.
When an ABR receive the LSA Type 1 and LSA Type 2 within the area, it will only send the reachability information through the LSA Type 3 to another area. ABR hides the topology information and only reachability information sends between the areas.
To prevent routing loops, areas must be connected to the backbone area 0. All LSAs Type 3 must therefore pass into or out of area 0 when multiple areas are in use, whereas type 1 and 2 LSAs are confined to the local area. In other when we have multiple ABRs, an ABR ignores an LSA Type 3 learned through a non-backbone, this called as split-horizon inter-area loop prevention.
Question 3: what is the command to stop LSA-7?
In NSSA, external routes enter as LSA-7. But, if LSA-7 is not required, what is the command to stop LSA-7 ? if you need an NSSA area but you do not want to advertise a Type-7 LSA, to stop a Type-7 LSA to traverse the NSSA area, use the “nssa-only” keyword in the redistribute command. The “nssa-only” instructs the ASBR to clear the P-bit in its Type7-LSA. The P-bit = (P – propagate) is only used in type-7 LSAs to tell the ABRs to translate that LSA Type 7 into an LSA Type 5.
Question 4: How the NSSA ABR translator election occurs?
We have one ASBR & two ABR’s. which ABR will do LSA-7 to LSA-5 conversion? the highest ABR router ID will be the translator per RFC 1583 and 3101, but if you want to force an ABR to be a translator regardless the router ID selection, make sure the RFC is 3101 enabled (this is the default) and execute the area X nssa translate type7 always command.
Question 5: How to stop receiving specific subnets In NSSA?
While receiving routes through LSA-7, can we receive selected subnets or stop a few subnets from receiving? you can control the advertised routes through Type-7 using the summary-address command with the “not-advertise” keyword, the component networks remain hidden from other networks. or using the route-map associated with the redistribute command.
Question 6: What is the LSInfinity?
LSInfinity in OSPF has two two different values, defined in two different RFCs 3137 and 2328 but two different meanings.
- 16-bit (65535) for Router LSA
- 24-bit (16777215) for Summary LSA and External LSA.
However, the key point here is that LSInfinity has two different meanings:
For Summary and External LSAs, LSInfinity 16777215 means "unreachable".
For Router LSAs LSInfinity 65535 means "least desirable". It ensures other routers to use other Type-1 LSAs stored in their LSDB. it’s possible to have a valid route with an LSInfinite cost.
Shutdown OSPF Stub Router Advertisement is defined in RFC 3137 in 2001, updated by RFC 6987 in 2013 ---> updated by RFC 8770 Host Router Support for OSPFv2 in 2020 with a new bit called Host bit H bit.
Question 7: In OSPF, why E1 routes are preferred over E2 routes?
E1 route uses lowest redistributed cost + lowest cost to reach ASBR we have hot potato + cold potato routing so packet will reach destination as quickly as possible. E1 considers only the redistributed metric and ignores the internal metric to reach ASBR, this behavior is called cold potato routing.
Question 8: In OSPF, what is forwarding address?
The Forward Address is a concept related to external route through a Type-5 under certain conditions and through a Type-7 (mandatory). The idea behind this FA is to avoid a suboptimal routing in some design to reach the external routes, in other words it helps a router in an area to have a view of a hidden topology in other area, for example in NSSA, the ASBR is no longer visible from other areas, this can cause a suboptimal routing in the OSPF domain to reach the external subnets from other areas, and to make the ASBR visible, the Forward Address which is an IP address that belongs to the ASBR will help other routers in other areas to locate and calculate the shortest path to the ASBR through an inter-area route toward this Forward Address, thus the best path to the external subnets.
Question 9: What is the partition of the backbone area 0?
The answer is in RFC 2328 3.7 section.
RFC 2328 says:
3.7. Partitions of areas
OSPF does not actively attempt to repair area partitions. When
an area becomes partitioned, each component simply becomes a
separate area. The backbone then performs routing between the
new areas. Some destinations reachable via intra-area routing
before the partition will now require inter-area routing.
However, in order to maintain full routing after the partition,
an address range must not be split across multiple components of
the area partition. Also, the backbone itself must not
partition. If it does, parts of the Autonomous System will
become unreachable. Backbone partitions can be repaired by
configuring virtual links (see Section 15).
A discontinuous of the backbone area can result in undesirable result of losing external route when we have multiple ABRs and you are redistribution into a backbone area instead of the non-backbone area, the undesirable result is caused by the loop prevention mechanism of Type-4 LSA between ABRs.